Process control systems require the accurate measurement of process variables. Typically, a sensor in the form of a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal representing sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few.
In one form, a through air measurement instrument, such as a microwave radar level transmitter, launches a radar signal which reflects off a liquid or other surface and the instrument measures time of flight between transmission and reception of the radar signal. Electrical energy is converted to an electromagnetic wave from a launch element. The wave propagates through free space.
A two-wire transmitter includes two terminals connected to a remote power supply. The transmitter loop current, drawn from the power supply, is proportional to the process variable. A typical instrument operates off of a 24 volt DC power supply and varies the signal current in the loop between 4 and 20 milliamps (mA) DC. Thus, the instrument must operate with current less than 4 milliamps.
While low power circuits are continuously developed, there are other increasing demands placed on performance capabilities of the process control instruments. For example, with a radar level measurement device, the instrument's performance is enhanced by more powerful digital signal processing techniques driven by a microprocessor. In addition to the microprocessor, there are several other circuits, such as the radar transceiver, which requires electric power. To be successful, the instrument must use optimum processing capability and speed. This means making maximum power from the loop available to the electronics, and using it efficiently.
More recently, the loop powered instruments have utilized digital communications. Typical digital communications rely on two-way communication signals. The communication into a typical sensor device is by voltage level modulation. The communication out of a typical sensor device is by modulation of the current draw of the unit. In normal operation, the instrument must allow for 4 mA to 20 mA loop current while still communicating digital signals via modulation of the supply voltage and loop current. In addition, it is necessary to maintain high input impedance for digital communications.
In applications where a device must satisfy explosion proof requirements, a galvanic isolation circuit may be provided. However, such an isolation circuit can cause a problem with modulation of the supply voltage. Digital communications require a high input impedance into the level measuring instrument. Unfortunately, with the galvanic isolation circuitry, the communication input voltage modulated signals are not reliable once they are received at the modem through the instrument's traditional power line connections.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.